My roommate and I were in a heated debate that lead us to read your post about the ability to survive the end of Portal 2. However, our question is slightly different. Suppose the same kind of portal was created on Earth’s surface to the Moon’s, how long would it take for the Earth’s air supply to be released through the portal into space?

At higher and higher altitudes, the Earth's atmosphere becomes so thin that it essentially ceases to exist. Gradually, the atmospheric halo fades into the blackness of space. This astronaut photograph captured on July 20, 2006, shows a nearly translucent moon emerging from behind the halo. Image credit: NASA

If any of you haven’t seen the previous Portal 2 post, I’d recommend having a look at it here, because I’m going to pull some numbers from it. I’m also going to make some slightly unphysical assumptions, but the results of those assumptions is that we’re going to calculate a lower limit to the amount of time it would take to bleed the atmosphere dry. In a world where portals actually worked, it would almost definitely take longer, for reasons we’ll go over later.

Our scenario is thus: we have opened a portal between the surface of the Earth and the Moon, as in the end of Portal 2. Effectively, we’re opening a window between the surface of the Earth and a pretty hard vacuum. The dramatic pressure difference here produces a tremendous, faster than the speed of sound, wind, as we worked out in that previous post. Presumably, if you left that portal open for a long time, you would reduce the amount of atmosphere left on the Earth. In the game, this portal is only open for about 30 seconds, but what if we left it permanently open?

The first thing I’m going to assume is that the whole atmosphere of the Earth is entirely at the same pressure (which it is not). Down at the surface where we humans live, the atmosphere is pretty compressed, and so we have an ambient atmospheric pressure of 1 atmosphere. (Yep. That’s the unit.) 1 atmosphere is equivalent to about 14.7 pounds per square inch, or psi. However, the further up away from the surface you go, the more diffuse the atmosphere gets, and both the density of atoms and the atmospheric pressure drops. If the density of the atmosphere drops, the wind speed through our window will also drop, because it’s the difference in pressure on the two sides of our window that drives the wind speed. By assuming that I can compress down the upper layers of the atmosphere so that the air on Earth is at a constant 14.7 psi, then the wind speed will stay at its fastest, and bleed the atmosphere out into Moon space as fast as possible.

I will have a reasonable guess that the portal itself is about five feet tall by three feet wide - it seems a bit shorter than Chell in game, and wide enough for her to fit through. If we assume that it’s rectangular instead of an oval, the math is nicer, so I’m going to square up the portal dimensions at about 1.5 meters high by 1 meter wide. This gives a portal area of 1.5 square meters. This is key, because with the area of the window, and the wind speed, we can figure out the volume of air lost every second. At 411 meters per second, our speed from the older post, that means that after one second, a bit of air will have traveled 411 meters.

Every second, we’re going to lose about 617 cubic meters of high pressure Earth atmosphere into the space surrounding the Moon. We know how much we have to lose, so from here we can sort out how many seconds it would take to get the total volume of the Earth’s atmosphere out through our portal. As you can probably guess by the 18 zeros following the total volume of the Earth’s atmosphere, it’s going to be a lot of seconds. In fact, it’s so many seconds that seconds are not a useful unit even a little bit. Converting into years is a little better.

It would take 215 million years.

Most ISS images are nadir, in which the center point of the image is directly beneath the lens of the camera, but this one is not. This highly oblique image of northwestern African captures the curvature of the Earth and shows its atmosphere. Image credit: NASA/JPL/UCSD/JSC

And remember, this is assuming that the wind speed stays the same the whole time, which it would not in real life. The other thing we’re assuming is that none of this gas will hang around the moon and increase the atmospheric pressure around the Moon. That would also start to balance out the pressure difference, slowing the wind speed down and making this take even longer. The moon historically is not very good at holding onto an atmosphere, so this would likely be a temporary arrangement, but millions of years is not very long for astronomical things, and it’s possible the lunar atmosphere could hang around long enough to slow down our wind. The estimates for the atmosphere around the young moon is that it would have stuck around for 70 million years or so - shorter than our fueling time, but long enough that we could expect it to hang around for a while, before we’re able to finish emptying the Earth’s atmosphere into outer space.

In reality, there would likely be an equilibrium point reached, where both the Moon’s newfound atmosphere and the Earth’s freshly drained atmosphere would find themselves at the same pressure, and the wind, having gradually slowed, would come to a stop, with only the vaguest breeze from the Earthward side as the Sun gradually stripped the atmosphere from around the Moon.

If supermassive black holes are so bad at feeding from the stars around them, how did they get so big?

This illustration shows a glowing stream of material from a star, disrupted as it was being devoured by a supermassive black hole. The feeding black hole is surrounded by a ring of dust.Image credit: NASA/JPL-Caltech

This is such a good question that the answer is not particularly settled yet - we have some ideas for how some supermassive black holes may have gotten so large, but it’s not clear that the explanations we’ve come up with so far hold true for all supermassive black holes.

So a few definitions before we launch into our ideas - by and large, all massive galaxies have a gigantic black hole at their centers. To distinguish these central black holes from the type of black hole created when a single, massive star ends its life, we named the big ones “supermassive”. The black holes created from single stars are less entertainingly named “stellar-mass black holes”.

This artist concept of the local galaxy Arp 220, captured by the Hubble Space Telescope, shows the bright core of the galaxy, paired with an overlaid artist's impression of jets emanating from it, to indicate that the central black hole's activity is intensifying. Image Credit: NASA/JPL-Caltech

Black holes of all sizes are extremely inefficient at gathering new material to themselves. Rather than absorbing nearby material in one fell swoop, the material is more often than not pulled into a tight disk of extremely hot material, or flung away from the black hole entirely, sometimes at relativistic speeds. For all black holes have a reputation of being cosmic garbage disposals, if you had a garbage disposal this terrible in your own kitchen, after the first time it blasted superheated onion bits on your ceiling you’d call a professional to have it removed ASAP.

What can we do? Well, we can either 1) start larger, or 2) grow differently. If you start larger, then you have the benefit of not having to grow by a factor of several thousand but only by a couple. This method means you have to start with “seed black holes” very early in the universe. You can do this in one of two ways. First is the direct collapse model - the thinking goes that it might have been possible, very early in the universe for enough gas to collect together that its gravity would just collapse all the way down to a black hole, skipping the star phase entirely. The second method is effectively to go through a star first, but to go through a very, very large star - something much larger than the Universe makes nowadays, which would burn through its hydrogen much faster, and would make a larger black hole as a remnant.

There’s no reason the answer will wind up being one or the other - some combination is likely to be in play. If you can start with a larger seed in the early Universe, you can grow more easily through a combination of colliding with the supermassive black holes in other galaxies, and by gathering gas inefficiently to themselves.

VLT NACO image, taken in the Ks-band, of GQ Lupi. The feeble point of light to the right of the star is the newly found cold companion. It is 250 times fainter than the star itself and it located 0.73 arcsecond west. At the distance of GQ Lupi, this corresponds to a distance of roughly 100 astronomical units. North is up and East is to the left.Credit: ESO

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Our list of known planets and exoplanets unfortunately doesn’t extend much beyond our own Milky Way galaxy - to spot a planet, you need to be able to measure the light from an individual star and monitor it over time. You’re looking either for tiny flickers in the amount of light you receive, as a planet happens to pass in front of the star you’re watching, or you’re looking for there to be a little Doppler shift in the color of the star’s light, as the planets tug it slightly off center as they orbit. Known by the names of the transit method and the Doppler shift method respectively, both of these require really careful observations over a significant amount of time, without the light from the star mixing with the light from other stars. This limits us pretty well to the stars within or surrounding our Milky Way.

Because the measurements required to spot planets must be so precise, generally the telescopes we send out to do these measurements only look at a small patch of the sky. So while I can give you our current high scoring planets, there’s no guarantee these will remain the all-time bests, if we point our telescopes in a new direction.

There is one fundamental limitation to how massive a planet can get - if you pack too much material into a planet, it will start to fuse elements in its core, and it formally becomes a star instead of a planet. This transition happens when the object is somewhere in the range of 13 to 80 times the mass of Jupiter, and is the point at which we typically start calling objects a brown dwarf star, orbiting another star, instead of a planet. The list of biggest planets can also change if we get better measurements. It's possible to learn that what we thought was a planet should really be called a brown dwarf, which then bumps that object off the list of biggest planets, and onto the list of known brown dwarfs.

This artist's conception illustrates what a "Y dwarf" might look like. Y dwarfs are the coldest star-like bodies known, with temperatures that can be even cooler than the human body. Image credit: NASA/JPL-Caltech

However, you can still have very large, fluffy planets, well before they get to this boundary of being a star. Most of the ones we know about are Jupiter like in style - massive, gaseous planets, orbiting distant stars. The easiest to find are hot Jupters - exoplanets which are not only bigger than Jupiter, they’re much closer into their star than Jupiter is to our Sun. Currently, the majority of the biggest, fluffiest planets are about twice the radius of Jupiter. Considering that you could stack 22 and a half Earths edge to edge to match the width of Jupiter, you’re looking at a planet so large, you could line up 45 Earths behind it, and not see any of them. These planets have the very pronounceable names of ROXs 42Bb, which is estimated to be about 2.5 times the size of Jupiter, or Kepler-13 Ab, which sits around 2.2 times the size of Jupiter.

There are some larger ones, but these have preliminary estimates of their size, and may yet turn out to be brown dwarfs. The current record holder is a planet orbiting a star known as GQ Lupi, and estimates place it at somewhere around 4 times larger than Jupiter. This particular object is so large that our theoretical models of how it has formed are not particularly happy, and so the estimates on its size and mass are both pretty hazy. It is likely to remain a planet, but if it turns out that its mass is on the high end of our current estimates, it could wind up on a brown dwarf list. (This object is also extremely young, and will change and compress as it evolves.)

These big fluffy planets are orbiting your default solar system - one with a single star, around which all the planets orbit. If you have two stars (which isn’t that uncommon), it seems to be much harder to build very large planets. The largest planet known to circle two stars at once was only confirmed in 2016, and is almost identical to Jupiter in size. At “only” 22.5 Earths in size, it orbits its parent star once every three years.

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If it is impractical to provide an artificial magnetosphere on the ship which would travel to Mars (due to cosmic ray cascades in the material of the ship), what about generating the magnetic fields externally and projecting them into space at a series of waypoints? Or would the distance involved (225 million miles) be too great?

Generally we want the outer walls of our spacecraft to be pretty durable, both for airtightness, protection against space junk, and to help protect against the solar wind, which can be stopped by a pretty reasonable amount of shielding. However, as you build up your shield, cosmic rays will start to play a nastier role. While you certainly don’t want a cosmic ray to be able to pass straight through your spacecraft and hit your astronaut unhindered (they’re very energetic particles, the sort that bodies deal very badly with), when a cosmic ray hits a dense object like a wall, it doesn’t just bounce back the way it came from.

It creates a radiation cascade instead; what was one particle is now two, four, sixteen, and beyond, very rapidly, as the particle interacts with the dense material of the spacecraft wall. Sixteen slightly lower energy particles is mathematically worse than one high energy one, and a serious point of concern once we get out of the Earth’s magnetic shielding. So a very reasonable response is to ask if we can bring along our own magnetic shielding, to prevent the high energy cosmic rays from hitting the wall of the spacecraft in the first place. Theoretically, this should reduce the amount of radiation inside the spacecraft cabin, since it would reduce the number of cosmic rays that can make it all the way to the spacecraft shield. The main reason this is impractical right now is simply a logistical one - we don’t have a good way to build a generator for a sufficiently strong magnetic field which is also lightweight enough not to be hard to launch.

Setting up waystations would be an interesting way of approaching the same challenge. If there were a fixed orbital path between the Earth and Mars, and we could build a magnetic tube between the two planets, you could do away with the need to have an onboard magnetic bubble. Because you’re not trying to launch them on the spacecraft, you wouldn’t need to worry about the weight as much, but the magnetic field you’d have to generate would need to be much larger, to guarantee that the spacecraft (within errors) would definitely travel safely through the buffered region. The distances involved here are vast, and so setting up a series of waypoints would almost definitely be unfavorable, at least from an energy consumption perspective. There’s also the question of fueling those waypoints. Are they solar powered? Fission powered? What happens if their solar panels break down or they run out of energy? They’d also have to be able to correct their own orbits in order to be in the right places for the protection of the traversing spacecraft, and at this point we’re looking at a giant electromagnet with rockets, which is a great sounding device to have, but practically speaking, it’s a more powerful version of what we’d like to have on the spacecraft in the first place, and if we can get by with one device instead of several hundred, one is probably better.